Endress+Hauser Tophit CPS471, Tophit CPS471D Technical Information

Technical Information

Tophit CPS471 and CPS471D

Sterilizable and autoclavable ISFET sensor for the pH measurement
Analog or digital sensors with Memosens technology

R

StandardNumber 74-02

0

1

Application

• Hygienic and sterile applications

• Food industry and pharmaceutical industry

• Biotechnology

With ATEX, FM and CSA approval for application in hazardous
areas

Your benefits

• Resistant to breaking
– Sensor body made completely of PEEK
– Direct installation into the process, reduces effort and costs

for sampling and laboratory analysis

• Certified biocompatibility

• Double-chamber reference system:
– poisoning resistant
– polyacrylamide free gel

• Application possible at low temperatures
– Short response time
– Constantly high accuracy

• Sterilisable and autoclavable

• Longer calibration intervals than glass electrodes
– Lower hysteresis with alternating temperatures
– Low measuring error after high-temperature loading
– Almost no acid and alkaline errors

• With built-in temperature sensor for effective temperature
compensation

• Ideal for CIP processes when combined with an automatic
retractable assembly

Further benefits offered by Memosens technology

• Maximum process safety through contactless inductive signal
transmission

• Data safety through digital data transmission

• Easy handling due to storage of sensor-specific data

• Predictive maintenance possible thanks to registration of
sensor load data

TI283C/07/en/04.06
51506685

Tophit H CPS471/CPS471D

Function and system design

Measuring principle Ion-selective, or more generally ion-sensitive field effect transistors (ISFET) were developed in the 1970s as

an alternative to the glass electrode for pH measurement.

Basics

Ion-selective field effect transistors use an MOS
(pos. 1) is not a control electrode. Instead, the medium (→ å 2, pos. 3) in the ISFET is in direct contact with
the gate isolator layer (pos. 2).Two strongly N-conducting areas are diffused in P-conducting substrate
(→ å 2, pos. 5) of the semiconductor material (Si). These N-conducting areas are current supplying
("Source", S) and current accepting ("Drain", D) electrodes. The metallic gate electrode (in case of the
MOSFET) resp. the medium (in case of the ISFET) forms a capacitor with the substrate below. A potential
difference between gate and substrate (U
N-conducting channel (pos. 2) is formed, i.e. a drain current (ID) is induced.

1)

transistor arrangement (→ å 1) where the metallic gate

) causes a higher electron density between "Source" and "Drain". A

GS

13

U

U

GS

Si (n)

S

Fig. 1: Principle MOSFET

1 Metallic gate
2 N-conducting channel

2

Si (p)

U

1

Si (n)

D

I

D

D

a0003855

GS

6

Si (n)

S

5

Fig. 2: Principle ISFET

1 Reference electrode
2 N-conducting channel
3 Gate isolator layer
4Medium
5 P-doped silicon substrate
6 Sensor shaft

2

With the ISFET, the medium is in direct contact with the gate isolator layer. Therefore, H

Si (p)

U

D

+

ions available in the

4

Si (n)

D

I

D

a0003856

medium, which are located in the medium / gate isolator boundary layer, create the electric field (gate
potential). Depending on the effect described above, a N-conducting channel is formed and a current between
"Source" and "Drain" is induced. Suitable sensor circuits use the dependence on the ion-selective gate potential
to create an output signal proportional to the concentration of the ion type.

+

pH selective IsFET The gate isolator serves as an ion-selective layer for H

well (isolator effect) but allows reversible surface reactions with the H
Depending on the acidic or alkaline character of the measurement solutions, functional groups in the isolator
surface accept or reject H

+

ions (amphoteric character of the functional groups). This leads to a positive (H+

acceptance in the acidic medium) or negative (H

ions. The gate isolator is impermeable to the ions as

+

rejection in the alkaline medium) charging of the isolator

+

ions.

surface. Depending on the pH value, a defined surface charge can be used to control the field effect in the
channel between "Source" and "Drain".The processes which lead to the creation of a charge potential and
therefore to a control voltage U

UGS ...
U
R ...
T ...
n ...

Potential between gate and source

...

Offset voltage

0

Gas constant (8.3143 J/molK)
Temperature [K]
electrochemical valueability (1/mol)

between "Gate" and "Source" are described with the Nernst equation:

GS

F ...
a

ion

Faraday constant (26.803 Ah)

...

Activity of ion kind (H+)

Nernst factor

At 25 °C (77 °F), the Nernst factor is –59.16 mV/pH.

1) Metal Oxide Semiconductor

2 Endress+Hauser

Tophit H CPS471/CPS471D

Important characteristics of Tophit CPS 471

• Resistance to breaking
This is the most obvious feature of the sensor. The complete sensor technology is embedded in a PEEK shaft.
Only the highly resistant isolator layer and the reference have direct contact with the medium.

• Acid or alkaline errors
A further, important benefit compared with the glass electrode is the considerably reduced number of acid
or alkaline errors in extreme pH ranges. In contrast to glass electrodes, practically no foreign ions can build
up at the ISFET gate. The measuring error of < 0.01 pH (between pH 1 and 13) at 25°C (77 °F) is near by
the detection limit.
The figure below shows the acid or alkaline error of the ISFET between pH 1 and 13 and the comparison to
the glass electrode (two different pH glasses) at pH values 0.09 and 13.86.

DpH

E+H ISFET

0.8

Glass 1

Glass 2

0.6

0.4

0.2

0.0

-0.2
pH1…13

Fig. 3: Comparison of acid and alkaline errors

• Measurement stability and sensor response time
The ISFET response times are very short over the whole temperature range.
With the ISFET sensor, there is no (temperature-dependent) equilibrium setting as in the source layer of a
pH glass of a glass electrode. They can also be used at low temperatures without a deceleration in response
time.Large and fast temperature and pH value fluctuations have a smaller effect on the measuring error
(hysteresis) than with a glass electrode, as there is no stress exerted on the pH glass.

• Reference system
The integrated reference electrode of the sensor is a double-chamber reference system with a bridge
electrolyte. The benefits are an efficient and stable contact between the diaphragm and the reference lead,
and the extremely long poisoning path. The bridge electrolyte is highly resistant to temperature and pressure
changes.

1 M NaOH1 M HCl

pH (25 °C)

a0003867-en

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Tophit H CPS471/CPS471D

• Isothermic curves
– The Nernst equation defines the dependence of the measuring voltage on the hydrogen ion content (pH

value) and the temperature. It is the basis of pH measuring technology and for ISFET sensors too. A
temperature-dependent value for the potential change per pH value can be worked out from this equation
(isothermic curve, potential change per pH value at a defined temperature).

– The isothermic curves of the ISFET sensor are very close to the theoretical values (→ å 4). This is further

proof for the high pH measurement precision of the sensor.

mV

500

300

100

-100

-300

-500
0246 8101214

a

b
c

Fig. 4: Isothermic curves

a Isothermic curve at 8 °C (46 °F), slope –55.8 mV/pH
b Isothermic curve at 37 °C (99 °F), slope –61.5 mV/pH
c Isothermic curve at 61 °C (142 °F), slope –66.3 mV/pH

pH

a0003868-de

4 Endress+Hauser

Tophit H CPS471/CPS471D

Memosens (CPS471D) Maximum process safety

The inductive and non-contacting measured value transmission of Memosens guarantees maximum process
safety and offers the following benefits:

• All problems caused by moisture are eliminated.
– The plug-in connection is free from corrosion.
– Measured value distortion from moisture is not possible.
– The plug-in system can even be connected under water.

• The transmitter is galvanically decoupled from the medium. The result: No more need to ask about
"symmetrically high-impedance" or "unsymmetrical" (for pH/ORP measurement) or an impedance
converter.

• EMC safety is guaranteed by screening measures for the digital measured value transmission.

Data safety through digital data transfer

The Memosens technology digitalizes the measured value in the sensor and transfers it to the transmitter via a
contactless connection. The result:

• An automatic error message is generated if the sensor fails or the connection between sensor and transmitter
is interrupted.

• The availability of the measuring point is dramatically increased by immediate error detection.

• The digital signals are suitable for application in hazardous areas; the integrated electronics are intrinsically
safe.

Easy handling

Sensors with Memosens technology have integrated electronics that allow for saving calibration data and
further information such as total hours of operation and operating hours under extreme measuring conditions.
When the sensor is mounted, the calibration data are automatically transferred to the transmitter and used to
calculate the current measured value. Storing the calibration data in the sensor allows for calibration and
adjustment away from the measuring point. The result:

• Sensors can be calibrated unter optimum external conditions in the measuring lab. Wind and weather do
neither affect the calibration quality nor the operator.

• The measuring point availability is dramatically increased by the quick and easy replacement of precalibrated
sensors.

• The transmitter does not need to be installed close to the measuring point but can be placed in the control
room.

• Maintenance intervals can be defined based on all stored sensor load and calibration data and predictive
maintenance is possible.

• The sensor history can be documented on external data carriers and evaluation programs at any time. Thus,
the current application of the sensors can be made to depend on their previous history.

Communication with the transmitter

Always connect digital sensors with Memosens technology to a transmitter with Memosens technology. Data
transmission to a transmitter for analog sensors is not possible.

The sensor is connected to the cable connection (CYK10) without contact. The power and data are transferred
inductively
Once connected to the transmitter, the data saved in the sensor are read digitally. You can call up these data
using the corresponding DIAG menu.

Data that digital sensors save include the following:

• Manufacturer data
– Serial number
– Order code
– Date of manufacture

• Calibration data
– Calibration date
– Calibration values
– Number of calibrations
– Serial number of the transmitter used to perform the last calibration

• Operational data
– Date of commissioning
– Hours of operation under extreme conditions
– Number of sterilizations
– Data for sensor monitoring.

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Measuring system The complete measuring system comprises at least:

• ISFET sensor Tophit

• Measuring cable CPK12 (analog, with TOP68 connection) or CYK10 (digital, with Memosens)

• Transmitter, e.g. Liquiline M CM4x, Liquisys M CPM223 (for panel mounting) or Liquisys M CPM253 (field
instrument) or Mycom S CPM153.

• Immersion, flow or retractable assembly, e.g. Cleanfit P CPA471 (CPA450 with digital sensor only)

There are additional accessories available depending on the application:

• Topclean S CPC30 or Topcal S CPC310 automatic cleaning system

• Extension cable, VBA, VBM or RM junction box

Food industry and chemicals

Tophit H CPS471/CPS471D

Fig. 5: Measuring system with Topcal S fully automatic measuring, cleaning and calibration system

Power cable

Tophit

1

Cleanfit H CPA475

2

Mycom S CPM153

3

Power supply

4

Control unit CPG310

5

Cleaner, buffer solutions

6

Steam, water, cleaner

7

Rinse block

8

9

Compressed air

10

Liquids / cleaner

11

a0003873

6 Endress+Hauser